Gyrator type resonators are widely used to implement poly-phase filters on integrated circuits. For example, see Integration of Analog Filters in a Bipolar process. J. O. Voorman, W. H. A. Brüils and P. J. Barth, IEEE Journal of Solid-State Circuits, Vol. SC-17, No. 4, August, 1982. Their symmetrical construction makes them well suited to filtering low intermediate frequency filtering in receivers using both in-phase and quadrature-phase signals that provide low signal distortion due to the advantages of the well known image rejection and the symmetrical (around the resonance frequency) frequency responses of both the amplitude and group-delay. For example, see U.S. Pat. No. 4,193,033.
Some conventional filter implementations of the gyrator type resonator use a combination of resistors and transconductors to tune the damping and hence the bandwidth. For examples, see U.S. Pat. No. 5,220,686 or patent application WO 02/087071 A3. Tolerances and temperature dependencies of the integrated resistors, capacitors and transconductors biasing circuitry all have their effect on the filter parameters, such as center frequency, bandwidth, shape and gain. Several solutions exist to counter this alignment problem. In one example (see Datasheet TEA6850, Philips Semiconductors, July, 1994), two potentiometers need hand alignment to set the center frequency and the bandwidth. It will be evident that hand tuning is not acceptable for high volume products due to cost considerations.
A second known solution is to add separate control loops on the receiver integrated circuit. In A wideband tunable CMOS channel-select filter for a low-IF wireless receiver. F. Behbahani, W. Tan, A. Karimi, A. Roithmeier, and A. A. Abidi. Custom IC Conf., San Diego, pp. 501-504, May 1999, a channel-select filter is described. A complex mixed analog-digital automatic frequency control loop is used to tune the center frequencies of the resonators in the filter. On top of that, a second mixed analog-digital loop is required to tune the Q of the filters.
The multiple loop calibration requirement is also apparent in some products currently on the market. S. Sandee and G. van Werven (Application Note, AN 00001, version 1.2. Philips Semiconductors, Jun. 26, 2000), for example, describe a radio with circumstantial controlled selectivity wherein a 7 bit digital to analog converter (DAC) is used to calibrate the center frequency, the bandwidth is dynamically controlled using an analog loop and the gain is calibrated using a 4 bit DAC. In another current product, the TEAS5767HL (see Datasheet TEA5767HL, Philips Semiconductors, Sep. 13, 2002) shows a low intermediate frequency filter that requires two separate alignment loops, one for the center frequency and one for the gain. In addition, both loops of the TEAS5767HL require a pin and an external component. Each of these calibration loops requires a supply current, which requires additional chip area and, in some cases, requires additional interface pins and external components.
A third solution is to correct the process spread by using an external micro-controller. This approach is demonstrated in A Digitally Programmable Zero External Components FM Radio Receiver with luV Sensitivity, H. van Rumpt, D. Kasperkovitz, J. van der Tang. IEEE—ISSCC 2003 and in a part currently available on the market, see Datasheet TDA7513T, ST Microelectronics, June 2004. [10, 11]. In most products, micro-controllers have a specific function, such as polling interrupts, updating the display, controlling the modes of functions, or scanning a keypad. The introduction of micro-controlled calibration may place an undesirable load on the micro-controller along with the system bus that may impair the micro-controller's ability to perform its primary functions.
It is an objective of the invention to obviate these drawbacks so that poly phase type filters can be produced with a high production yield, using less chip area, less current consumption and no additional pins nor external components.